I would appreciate comments from someone skilled in op-amp design. I want to build a PWM to voltage circuit. I put together this design using a low-pass filter, a dual channel op-amp and a Mosfet pass transistor. I would like the minimum output to the fan to be adjustable, but do not know if the voltage divider at the buffer output is viable configuration.

Best regards
trident

Last edited by trident on Tue Dec 01, 2009 12:56 pm, edited 4 times in total.

1) You have a few odd value resistors in there (40.2k, 60.4k). You shouldn't need anything that precise for a circuit like this.

2) Do you really need such a large gate stopper resistor (R7)? Values of 100 - 1k are more typical.

3) I think you could implement the minimum voltage in a much simpler way by adding a bias voltage to the input instead of the fancy voltage divider. This could be done via the resistor R4 in the attached image of the input part of your circuit.

Actually, I suppose it would still take the same number of components once you make it variable, but doing it this way could allow you to remove the second op-amp.

I'm not an electronics engineer, but the cct we have is clearly different from yours. It does not use any 3V inputs - just the Ground, 12V and PWM inputs from the fan header, but it uses 4 transistors not two, and like yours, it has a variable resistor in it to enable you to control the fan speed at low PWM input.

Total component count is 14 - so less than yours despite having two extra transistors.

The 3V in trident's circuit is not externally supplied, it's derived from the 12V from the fan header. It is needed because the 4-pin fan spec says that the PWM output should be pulled up to no more than 5.25V, and recommends 3.3V.

pcy wrote:

...but it uses 4 transistors not two...

The triangular devices in the diagram are op-amps (integrated circuits containing perhaps dozens of transistors). They can be used in place of discrete transistors in many circuits, giving improved performance, but with increased cost. The two in trident's circuit are both part of a dual op-amp package, so they are only one physical device despite appearing as two in the diagram.

Probably a lot cheaper considering that an op-amp costs on the order of Â£0.50, whereas a transistor might cost around Â£0.05 (bought singly - they are much cheaper in bulk). Of course either is far too cheap for the difference to matter for a one-off DIY project.

1)You have a few odd value resistors in there2)Do you really need such a large gate stopper resistor?3)I think you could implement the minimum voltage in a much simpler way by adding a bias voltage to the input

1)I am using metal film resistors for a small temperature coefficient, less drift with temperature, in small lots 1% resistors cost about the same as 5%2)I am using the MOSFET in linear mode, so I believe 100K is appropriate3)I agree that that your example is more straight forward, but please consider this if you will. In your circuit R4 is in parallel with R3. From what I understand about low-pass filters, the cut-off frequency would increase considerably. I decided on 100K and 0.1uF for a cut-off of 16Hz, below the range of hearing. As to using both channels of the LM358, were it not in circuit, I would need to add resistors to disable the spare channel, so it works out about the same.

Thank you for your reply Peter

pcy wrote:

I have the cct diagram around if you are interested.

I would appreciate a copy of your circuit, could you post it or PM a copy to me? I considered purchasing one of your boards but have no method for sending funds in GBP. Will it ever be available in the U.S.A. ?

Electronics is a hobby for me, I understand simple circuit analysis and have designed a few digital circuits. I know enough about op-amps to know there is a lot more for me to learn. This project gives me a reason to dig deeper into the subject.

Best Regards
trident

Last edited by trident on Wed Feb 25, 2009 5:43 pm, edited 1 time in total.

Hello1)I am using metal film resistors for a small temperature coefficient, less drift with temperature, in small lots 1% resistors cost about the same as 5%2)I am using the MOSFET in linear mode, so I believe 100K is appropriate3)I agree that that your example is more straight forward, but please consider this if you will. In your circuit R4 is in parallel with R3. From what I understand about low-pass filters, the cut-off frequency would increase considerably. I decided on 100K and 0.1uF for a cut-off of 16Hz, below the range of hearing...

1)Ok, but I think it best to stick to E12 or E24 values unless you really need the precision. You can still use 1% resistors, but it makes things easier if you need to change parts in the future.

2) The gate stopper resistor is only there to damp oscillations that may occur between the MOSFET's input capacitance and stray inductance in the leads/track. A low value is sufficient for this. A very high value doesn't do any better, but it does slow the MOSFET down a lot.

3) It does affect the filter (R4 appears in parallel with C2, affecting gain as well as cut-off frequency. However, you don't really need to filter down to 16Hz, because the PWM signal is all >20kHz.

Anyway, you have made me become interested in the problem, so I knocked up this transistor-based version:
Minimum voltage is set by RV1. Q1 is a buffer and level shifter. Q2 inverts the signal to drive X1.

Instead of an RC low-pass filter at the input, C1 rolls off the gain of X1 at high frequencies. The effect is the same.

Anyway, you have made me become interested in the problem, so I knocked up this transistor-based version

Recently I ran across Maxim application note AN3149. Just now it occurs to me how similar it is to your discrete transistor circuit. I made some substitutions and derived this:

In this circuit the JBTs are common 2N3906 and 2N3904 transistors. If I replace R2 with a 5K resistor to +3 volts and 5K trimmer to J1 pin-4 and then connect R3 to the wiper, would it give equivalent function? To finish up, I will need to find worst case current draw and calculate a new value for R1. As to the relative merits, I would guess your circuit will reject noise from the fan motor better than the an3149 circuit, but I will need to find the appropriate transistors. I have all the transistors for the an3149 circuit in my spares box. What is your opinion?

Best Regards
trident

Last edited by trident on Sun Apr 25, 2010 4:49 pm, edited 3 times in total.

It looks pretty much identical, yes. I suppose that's because it's the obvious, simplest solution. Similar three-transistor amplifiers are used for all sorts of purposes.

If I understand what you describe for the resistor/trimmer, that will give you a range of 0-6V for the minimum voltage.

Having the low-pass filter at the input as in Maxim's circuit, instead of the feedback capacitor as in my circuit, should result in less EMI (the two BJTs are not switching) and better power supply rejection (the feedback capacitor couples noise on the 12V rail to the output) at the cost of one extra resistor.

Here is the updated circuit. Q3 will require a heat sink. Because current the draw of a brushless fan is approximately a linear function of applied voltage, worst case dissipation in the pass transistor can be estimated by: Pd = 1/4 ( V x I ), where V and I are the rated voltage and current of the fan. Depending on a number of factors, the value of R7 could need adjustment to get maximum fan voltage at 100% PWM input, but I can hash that out on breadboard. Thanks for the help Mr Evil.

Best Regards
trident

Last edited by trident on Sun Apr 25, 2010 4:49 pm, edited 3 times in total.

Hi, this is a very interesting topic. I will soon receive my Gigabyte motherboard and it may be that it has only PWM fan control, so I might be obliged to build one of these. I'm no expert when it comes to MOSFETs... would it be OK to replace the NDP6030PL with eg. IRF7416? Thanks in advance.

This circuit is not a done deal as yet. I have it on breadboard and in testing it with an INTEL M/B the fan header behaves as if it has an integral 4 volt pull-up, that is when the duty cycle is above 75% the low-pass filter outputs more than 3 volts. There are several ways to overcome this issue: reconfigure the feedback resistors, attenuate or clamp the PWM input voltage and a level shifter which I am currently working on. The final result I am looking for is "black box" that you plug in and it works, no tuning or fiddling required. As to your question:

ju1ce wrote:

... would it be OK to replace the NDP6030PL with eg. IRF7416?

I can not advise with any authority if the IRF7416 is a proper substitution. I decided to use the NDP6030PL because of the TO-220 package. I looked for a logic level P-channel enhancement mode MOSFET with low Vgs(th) for less offset. Vgss should be 12 volts or better. Id must be large enough to support the fan you intend to use. Don't use a high voltage MOSFET, go for a lower BVdss and Id to minimize gate capacitance.
If you are going to use an SMT device why not use the ZXM61P02 specified in the Maxim application note AN3149?
Seeing as how you are in Europe you may want to look in to the Nanoxia PWMX Controller. If they are any good please post a mini review.

Have been super busy as of late, this is the level shifter I've been working on. Trying to get parts ordered this week to test on breadboard. The minimum speed feature was removed from the input because it would distort the speed signal. R2 and R3 set default fan speed at full if the control signal fails open. If the control signal fails to ground fan speed will be zero. The output is a non-inverted 0 to 3 volt voltage proportional to the duty cycle of the speed signal.

You have made a mistake there - you can't have the RC filter after Q2 because the resistor will be in series with the fan, and thus have virtually all the output voltage dropped across it. If you want the filter at the output, make it an LC filter.

You'll also find that you can't control the default speed with R2/R3 because without negative feedback around the circuit, the gain will be very high.

The last circuit is only an input section. The 0 to 3 volt signal is not a final output. My apologies for the misunderstanding. I agree that the low-pass would need a buffer to prevent loading. Will R2/R3 function as intended using the the three transistor amplifier, shown earlier, to set a default fan speed? I would appreciate comments as how to best accomplish that. Irregardless a pull-up is required to adhere to the 4-wire fan specification. Would something like this work?
______
trident

Shuriken
I am curious to see what Nanoxia has put together. Would it be possible for you to post pictures of the component side and trace side the circuit board?

Mr Evil
Thanks for the reply, was thinking about adding a minimum speed feature to the amp section but it is not a priority at this time. Want to get a prototype put together ASAP to finish up current system build in progress.
______
trident

Last edited by trident on Sun Apr 25, 2010 4:50 pm, edited 1 time in total.

Thanks for posting the pictures Shuriken
My first impression is that Nanoxia is using a full-wave fan driver IC, because of the four transistors [Q1, Q2, Q3, Q4] that are driving the the two final output transistors [Q6, Q7]. D1 looks like a reverse polarity protection diode, U1 looks to be a three terminal voltage regulator. There also appears to be a low-pass [0.1uF, 100K].
______
trident

Legal stuff cover my arse paragraph:
Please be advised that if improperly constructed this project could cause costly and permanent damage to your computer system.
THIS INFORMATION PROVIDED ON AN 'AS IS' BASIS WITHOUT ANY WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED.

The NDP6030PL I wanted to use for the final output is no longer available in a TO-220 package, so a Sziklai pair is being used. This will limit the maximum output to Vcc - [~0.7v], but the whole idea here is to reduce the the fan speed and it's not a major issue for my application. At the other extreme, minimum output is about 1 volt. The TIP31 will need a heat sink. It can dissipate about 1 watt without one, but it will get hot enough to burn your fingers and the PC board.
______
trident

Hi all!
Didn't want to open a new thread, hopefully someone can answear me!
I want to line up 4 Arctic Cooling PWM fan, but my mobo has only one controllable PWM socket. I don't want to stress that socket with 4 fans, so i am wondering if i could do the following with them: i want to give power for the fans via the psu, and split the pwm signal to the 4 fans.
With this sollution will i be able to controll the fans with Easy Tune 6?
Thx in advance!

Arctic Cooling states that their 'PWM Sharing Technology' will support up to five Arctic Cooling fans so I do not foresee a problem with your idea. You will only be able to read the speed signal from one fan however.
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trident

i want to give power for the fans via the psu, and split the pwm signal to the 4 fans.

1)Use a fan-out cable similar to this ZALMAN [ZM-MC1], it has 5V and 12V taps, use the yellow 12 volt taps

2) replace the 3-wire connectors on the fan-out cable with 4-wire fan connectors
3) route the PWM signal to each 4-wire fan connector

Your motherboard manual would be the place to look for the power rating of the fan header. Four fans powered from a single fan header [pwm socket] could very well damage the motherboard.
_______
trident

Working on revised circuit, MOSFET selection not as critical(main requirements are Rds(on) @Vgs=10V: <100m Ohm and Vgss >12V).

edit 18 Apr 2010:
Experimenting with fixed integrator. The variable integrator has proven to be non-linear, due to a large difference in the charge and discharge currents and has been discarded. The revised circuit currently has no preset for a minimum floor voltage, that is 0% duty cycle gives near zero volts output. This circuit can therefor stop the fan. However using 1% resistors the integrator section is linear to ~2%.

Last edited by trident on Sat Apr 17, 2010 8:41 pm, edited 6 times in total.

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